Flames out the tailpipe!

It's been five years since my last major service on the BB, which included the obligatory carb rebuilds. Other than the annual "idle jet cleaning", she's been Honda-like in terms of reliability...until recently.

On a recent trip my friend, following me in his Ferrari, commented "how cool" the flames were shooting out of my right side tail pipes! I knew that she wasn't running quite right, as prior to that point, it felt like from time to time, she would drop a cylinder...and then, almost magically, run on all 12 again....perfectly. The problem came and went over a few weeks, and then finally, 11 cylinders prevailed, coupled with a lot of exhaust popping!

Here's the diagnostic process:

-Italian tune-up: HA! Sounds good, seems to be running on 12 at WFO, but drops back to 11 cylinders when puttering around
-Remove distributor cap and inspect: Clean, with no evidence of "green" Italian ignition fungus
-Pull 7-12 spark plugs (LF bank-RHS exhaust flames): All good except for #9: Very, very black and sooty
-Inspect idle/slow speed jet, fully expecting total blockage: Looks perfect!
-Ohm test ignition wires: Perfect
-Plug extenders (None on my car)

Other than a mechanical problem, which is unlikely with my "fresh" engine, the problem likely was in the carb itself. I pulled the carb, with the intention of doing a rebuild...only to find that all looked very clean and normal; with one exception.

The needle valve, which only controls the #9 cylinder on this particular carb, would stick at times...and not close completely when the float was at maximum height. The end result was an excessively high float level for this single barrel.

Fast forward: Carb rebuilt, needle valve changed...and a quickie carb set-up: She runs great.

Today's work: Pull the rest of the carbs to be safe, for rebuilds. If anyone is interested in photos or some step-by-steps...I'll post more information.

Oddly, in all the years of owning the BB, I've never had a needle and seat issue before. Perhaps all the ethanol in the fuel is finally taking its toll??

Regards,
David

 

Paul,

Weber's are "Italian" fuel injection, in its purest form....with all the sounds of glory that Enzo imagined. Their "quirks", if you will, are such a small price to pay for, considering the sedate performance achieved with that VW-based fuel injection system you have on your car....

Besides...Flames are cool!


David

 

Step 1-Using a 16mm socket, remove the needle and seat cover. These can be on extremely tight and it's much easier doing this whilst the carbs are still attached to the intake manifold. (More on this later)
Step 2-Loosen fuel line banjo bolts (18mm)
Step 3-Remove carbs
Step 4-Disassemble and clean

Note: The rebuild kits avaialble vary in quality and completeness, hence my collection of odd gaskets and seals is pretty large. For the most part, the kits are the same as the Porsche 911.

A few photos along the way...
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Nothing special here; just a pre-cleaning in my mineral spirits based parts cleaner, then a spray down with carb cleaner...then every passage blown out with compressed air. Then a re-cleaning with "Brake Parts Cleaner" to get rid of that last little bit of residue...and blown out again.

Cleaned body ready for reassembly...
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Fitting the main and auxiliary venturi's...

When installing the main venturi, note that the pointed set screw should align with the hole in the venturi , before tightening the screw down...
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As simple as this should be, they can be installed incorrectly, particularly on the middle barrel. The "hole" on the side of the aux. venturi always points towards the emulsion tube.
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Although there are four possible "holes" in each carb body, only three are used per carb. This is because all four bodies are identical, except that only some holes are used, depending on the carb placement on the engine.

Hence, the use of alu plugs present on some carbs, but not on others....
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With the main venturies, auxiliary venturi, emulsion tubes and air correctors in, it's time to fit the accelerator pump discharge check valves and nozzles. The check valves have little ball bearings inside of them that "must rattle" to function properly. After cleaning all the components, shake the check valve to verify the rattle before installation.

Note: There is only one gasket used (typically copper) under the assembled valve/discharge nozzle unit. With the "best fitting" screwdriver you can find, these valves should be installed as snugly as possible.
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Perhaps the most important part of the carb as this jet not only controls the idle mixture, but the entire slow speed mixture circuit. Almost all driveabilty concerns, including a rough idle or mid-range stumble/hesitation can be attributed to debris in this jet. When fitting, pay particular attention the o-ring, as it must be in good condition and fit completely recessed in the "hole", after the jet has been tighten. If the o-ring, or edges of it, are visible after the jet is tightened, the o-ring is too large...and vacuum leaks can occur, with corresponding "odd driveability" issues.

To ease installation, I spray the o-rings with a silicone lubricant...
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The main jets fit into a "holder" and can be removed with a 10mm socket and flat screw driver. They are the three "lower" brass caps on the carb bodies. The two upper brass caps are float bowl drains...or if you have the Weber "sight glass" tool, can be used for visual verification of the flow levels.
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Pretty straight forward re-assembly, just note that the "larger hole" on the inner pump diaphragm is on the bottom side of the housing, towards the base of the carb...
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The locating tab on outer pump housing points upwards....and fits into corresponding upper hole on carb body.
And the "button" on the outter pump diaphragm points outwards.
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This little guy, often called the pump by-pass valve, can cause all sorts of problems. There's only one per carb body, although there are two possible "holes" in the bottom of the float chambers where it could go. One of the holes is threaded, one is not...so it's location becomes obvious once you see it. The heart of this valve is a minute check valve (rattling ball). If it sticks open, you'll have a constant flow of fuel to the accelerator pump discharge nozzles....resulting in a very, very rich mixtures on all barrels. Remove, clean with solvent and compressed air. If she rattles, good to go. If not, re-clean or replace.
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These tapered needles serve to balance the individual air flows, barrel to barrel, in the same carb. Due to manufacturing tolerances, this is necessary to ensure equal air flow, at idle, through the three barrels.

Once removed, clean and inspect the needles. They should be sharp and carbon free. Reinstall until they just bottom in the carbs and lightly secure the locking nut. We'll be adjusting these later, once we get into the set-up and tuning process.

Important: Do not overtighten these needles, as permanent damage can be done to the carb bodies.
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Actually, they control the idle and slow speed mixture. For the BB only, Weber specifies that these (idle mixture) screws should be turned out approximately 3.5-4.0 turns.

Without going into too much Weber theory, if your mixture screws are turned out (in the above specified range)...and the progression holes in the carbs were drilled correctly (some were not), the hand off from the slow speed circuit to the main circuit should be smooth and free from any transitional stumbles. We all know that not to be true in most cases. Regardless, this is the ideal range for the 40 IF3-C carb (for both the 365 and 512).

Long and short: Before removing these screws, notice their relative positions and screw them in until they bottom in the carb body. If you're out more then 4 turns, you're too lean on the idle jet (as most BB's are today with the current fuel). Conversely, if you're out less than 3.5 turns, you're too rich on the idle jet and a smaller jet is in order.

As mentioned earlier, the idle jet controls both the idle and slow speed circuit. So, the position of these mixtures screws can have a dramatic effect on how the car idles and transitions....

To set a baseline: I've screwed them out 3.5 turns on all the barrels. We'll do the final set-up, once she running. Prior to installing these screws, lube the o-ring with silicone lube. Verify that the o-ring fits tightly on the threaded shaft!
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We're ready to install the floats and check the float heights. This is somewhat of a pain, but once they're set, they stay set. We'll get into this tomorrow.

David
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Prior to installing the floats, visually inspect them for damage or any evidence that they've been rubbing on the sides of the float chambers. There should be none. Inspect the float pivot pins for excessive wear/grooves. Light wear marks can be polished out. Anything more than that, the pins should be replaced.

Install the floats with the "tab" pointing up...And yes, they can be installed upside down! I prefer to use fibre washers on these pins, although copper is a reasonable substitute. Install the pins snugly, but do not fully tighten, just yet.

Using the Weber needle reference tool, set the needle depth to 18mm and attach it to the carb studs above the float, without the gasket in place. Here's the tricky part: You need to get the float tab resting against the protruding reference tool "plunger", without any excessive force (as we don't want to bend or stress) the float tab.

There are two way to do this:
-Use the Weber spring tool to hold the float up, or
-Invert the carb, so that gravity is holding the float tab against the plunger

Once the float is in the correct position (with the tab just resting against the pre-set plunger depth of 18mm), the distance from the top of the float to the top of the carb body should be 12.5-13.0mm without the gasket in place. If not, remove the float and adjust the tab accordingly.

Once this is done, secure the float pin and saftey wire in place.
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Fit a new top cover gasket...and install new nylocks, tightening in a diagonal pattern. These are 5.0x.8mm nylocks. Tighten to 4.5 lb-ft using a 1/4" drive torque wrench.
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